html {
  height:100svh;
  overflow:hidden;
}
body {
  margin:0px;
  width:100svw;
  height:100svh;
  background:linear-gradient(to bottom right,thistle,lightcyan) no-repeat left top;
}
.oper {
  float:left;
  font-size:2em;
  border:3px double #808080;
}
.oper:hover {
  border:5px solid #303030;
}
#con {
  color:red;
  border:none;
}
#d-b {
  display:grid;
  grid-template-columns:repeat(auto-fit,minmax(200px,1fr));
  position:absolute;
  top:0px;
  width:100%;
}
#d-a {
  display:none;
}